This invention relates generally to a gas discharge lamp and, more particularly, to an improvement of a gas discharge lamp which causes glow discharge by applying high frequency power between an internal electrode provided within a tube and an external electrode formed in a belt shape on the outer surface of the tube along the axis of the tube. Such a gas discharge lamp is disclosed, for instance, in Japanese Patent Disclosure No. 58-34560 (U.S. Pat. No. 4,645,979) which has one of the electrode pair provided within the tube as an internal electrode and has the other one of the electrode pair intimately provided on the outer surface of the tube as an external electrode. The internal and external electrodes are applied with high frequency power from an external power source so as to cause glow discharge within the tube.
The external electrode of the discharge lamp is a belt shaped electroconductive film or the like, which has uniform width along the axis of the tube. However, a uniform luminance distribution of the discharge lamp mentioned above along the axial direction of the tube is not obtained, which fact is found by no other than the present inventors for the first time. Although the reason for this non-uniform luminance distribution has not been clarified, the following can be deduced.
While the discharge lamp is being turned on, glow discharge occurs between the external and internal electrodes. The external electrode is formed in a belt shape along the axis of the tube, while the internal electrode is sealed in the tube and is positioned at one end thereof. Thus, the distance between both electrodes becomes larger towards the other end of the tube. Accordingly, the farther a position of the external electrode is from the internal electrode, the gentler the electric field strength becomes, which leads to a decreased density of ions and electrons, both obtained by electric dissociation. Accordingly, the larger the distance between these two electrodes becomes, the lower the current density becomes, which leads to lowering the ratio of the excitation of the fluorescent material. Thus, the end portion of the tube that is far away from the internal electrode will be low in luminous intensity compared with the central portion of the tube. A similar phenomenon will occur to a gas discharge lamp in which no fluorescent material is used.
At the proximity of the internal electrode or the region where the distance between both electrodes is small, the current density is higher than at the center portion of the tube. Since, however, a sufficiently large space can hardly be obtained at the proximity of the internal electrode, the electrons and ions will not be sufficiently accelerated. As a result, the excitation power of the fluorescent material will be small. Thus, the end portion of the tube that is near the internal electrode will be low in luminous intensity compared with the central portion of the tube.
Consequently, it may safely be said that the current density distribution greatly influences the luminance distribution.